Diffuser for light from light source array and displays incorporating same

ABSTRACT

An optical structure placeable between a backlight array of point light sources and a planar display. The structure distributes light emitted by the point light sources to uniformly illuminate the plane of the display, without introducing significant viewing parallax. The emitted light is partially collimated within a preferred angular viewing range, maximizing the display&#39;s luminance when viewed from the normal direction. The structure is highly reflective, such that a substantial portion of any non-emitted light rays are internally reflected by the structure, increasing the likelihood that those rays will be subsequently emitted by the structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/572,812 (accorded the filing date of 26 Jan. 2007), which is the U.S.National Stage of International Application No. PCT/CA05/00111 filed 15Jul. 2005, which claims the benefit of the filing date of U.S.provisional patent application No. 60/591,087 filed on 27 Jul. 2004.

TECHNICAL FIELD

This invention pertains to displays of the type which have an array oflight sources serving as a backlight. Light from the light sources isdistributed to achieve suitably uniform spatial and angular illuminationof the display, while maintaining high luminance in the display's normalviewing direction.

BACKGROUND

This invention pertains to backlights for displays which have atransmission-type light modulator illuminated by a backlight. Examplesof such displays include some liquid crystal displays (LCDs) as well ashigh dynamic range displays of the type disclosed in internationalpatent publication WO 02/069030 published 6 Sep. 2002 and ininternational patent publication WO 03/077013 published 18 Sep. 2003,both of which are incorporated by reference herein.

High dynamic range displays like those disclosed in the abovepublications incorporate a light source layer (which may be called a“backlight”) and a display layer that includes a light modulator. Thebacklight is controlled to produce a light pattern that represents acomparatively low-resolution version of an image to be displayed. Thelow-resolution image is modulated by the display layer to provide acomparatively high resolution image for perception by an observer.

The backlight typically comprises an array of point type activelymodulated light sources, such as light emitting diodes (LEDs). Thedisplay layer, which is positioned and aligned in front of thebacklight, may be a liquid crystal display (LCD) panel or the like.Maintenance of a relatively small separation distance between the twolayers allows light emitted by adjacent light sources of the backlightto merge smoothly into one another such that each pixel of the highresolution image is illuminated. Suitable image compensation techniquesmay be applied to remove undesirable image blurring artifacts.

In many planar illumination applications (e.g. not only in planardisplays as mentioned above, but also in some general illuminationsituations) it is desirable to uniformly illuminate (i.e. backlight) aplane. Multiple LEDs arranged in an array can be used in suchapplications since they provide a robust, low-power alternative toincandescent light sources. However, LEDs provide only point sourceillumination, not uniform planar illumination. It is consequentlynecessary to somehow distribute the light emitted by LEDs of a LED arrayso as to uniformly illuminate the plane.

In display applications it is also desirable to avoid parallax (apparentchanges in the direction of an object, due to changes in the observer'sposition which correspond to different lines of sight to the object)between each LED and the illuminated display area directly in front ofthe LED. Otherwise, an observer will perceive changes in that area ifthe area is viewed from different angles, which is undesirable.

The parallax problem has prevented attainment of uniform planarillumination in situations where each point type light source interactsin some manner with the display area directly in front of the lightsource, as is the case for LED/LCD type high dynamic range displays inwhich each LED corresponds to a specific pixel or cluster of pixels onthe LCD display. In such displays each LED should primarily illuminatethe LCD pixels directly in front of the LED. This illuminationcharacteristic should remain substantially invariant as the observer'sviewing angle changes.

It is also desirable that the light emitted by the backlight bepartially collimated within a preferred angular viewing range, namelywithin about 25° of the display's normal direction in order to maximizethe display's luminance when it is viewed from the normal direction. Itis additionally desirable that the optical structure as a whole (i.e.anything between the light source layer and the display layer) bereasonably reflective, in order to maximize the efficiency of thereflective polarizers incorporated in state-of-the-art LCD displays andthereby minimize light loss due to polarization.

SUMMARY OF THE INVENTION

This invention provides displays that include an optical structurebetween a backlight array of light sources and a display layer. Thelight sources may be point light sources such as light emitting diodes(LEDs). The structure distributes light emitted by the point lightsources. The invention also provides optical structures that may be usedto distribute light from arrays of point sources and related methods.

Various aspects of the invention and features of embodiments of theinvention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate non-limiting example applications ofthe invention.

FIG. 1 is a greatly enlarged not-to-scale cross-sectional side elevationview of a fragmented portion of a planar light distribution structure.

FIG. 2 is a greatly enlarged not-to-scale cross-sectional side elevationview of a fragmented portion of another planar light distributionstructure having an additional diffuser.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

FIG. 1 depicts a layered planar light distribution structure 10. Somelight sources 12, which may be LEDs, of a display backlight 13, and adisplay panel 14, which may comprise an LCD panel or other lightmodulator, are shown schematically in FIG. 1. Light distributionstructure 10 incorporates a rear reflector 16, a light-diffusing volume18, an optional reflective polarizer 20, and an optional angularlyselective light transmitter 22.

Rear reflector 16 has an array of transparent regions 24 at locationscorresponding to LEDs 12 of the backlight array. Transparent regions 12may comprise apertures or windows that are substantially transparent toat least some light emitted by LEDs 12. One transparent region 24 isprovided for each LED 12. Regions 24 are sized and aligned to match thesize and alignment of LEDs 12. For example, LEDs 12 may be arranged in arectangular array, a hexagonal array, a square array, or the like, andregions 24 may be arranged in a pattern that matches the arrangement ofLEDs 12. Regions 24 may also be shaped to match LEDs 12.

In the illustrated embodiment, LEDs 12 lack lenses. Such lensless LEDsemit light approximately in a Lambertian pattern (i.e. the emitted lighthas an intensity that varies with viewing angle in such a way that theintensity is directly proportional to the cosine of the viewing angle).In alternative embodiments, LEDs or other light sources may have lensesor may be otherwise constructed to emit light in a non-Lambertianmanner. Each region 24 transmits light emitted by the corresponding,immediately adjacent LED 12 into diffusing volume 18.

LEDs 12 may be outside diffusing volume 18, as shown. In thealternative, LEDs 12 could extend through regions 24, which may beapertures, and project slightly into diffusing volume 18.

The face 26 of rear reflector 16 that faces into diffusing volume 18 ishighly reflective. Face 26 is preferably at least partially specularlyreflective (i.e. the angle of reflection substantially equals the angleof incidence, in contrast to a diffuse reflector) and may besubstantially entirely specularly reflecting. Perforated “radiant mirrorfilm” available from 3M Specialty Film and Media Products Division, St.Paul, Minn. is one example of a material that can be used to form rearreflector 16.

The thickness dimension 28 of light diffusing volume 18 (which may be anair gap in the simplest case) is preferably minimized, while retainingsufficient thickness that light rays passing from rear reflector 16 intodiffusing volume 18 are asymmetrically diffused (i.e. light rays arescattered in many directions) before the rays pass through reflectivepolarizer 20. Consequently, any directional characteristic of light rayswhich enter diffusing volume 18 is essentially absent from light rayswhich exit diffusing volume 18.

Thickness dimension 28 can be reduced in cases where diffusing volume 18has an anisotropic scattering coefficient such that light rays which aresubstantially parallel to normal viewing direction 30 are more intenselyscattered than light rays which are substantially perpendicular tonormal viewing direction 30. Such anisotropic scattering can be achievedby placing, within diffusing volume 18, multiple layers of a thin,weakly light-scattering sheets 19 (See FIG. 2, not shown in FIG. 1).Partial reflection of light at surfaces of sheets 19 causes substantialscattering of light traveling in a direction substantially parallel tonormal viewing direction 30. Sheets 19 may be made of a suitabletransparent polymer material, for example. Anisotropic scattering couldalso be caused by providing within diffusing volume 18 a transparentmedium such as a suitable resin or gel doped with white particles (e.g.particles of paint pigment or the like) or other diffusely-reflectingparticles.

Optional reflective polarizer 20 (which has a polarizationcharacteristic matched to that of reflective polarizers incorporated inLCD display panel 14) reflects rearwardly into diffusing volume 18 lightrays having polarization characteristics which are unmatched to thepolarization characteristics of polarizers incorporated in LCD displaypanel 14. Such unsuitably polarized light rays undergo further diffusionwithin diffusing volume 18 and are again reflected (“recycled”) by rearreflector 16 toward optional reflective polarizer 20.

Diffusion within diffusing volume 18 randomizes the polarizationcharacteristics of recycled light rays such that some of the recycledrays are eventually able to pass through reflective polarizer 20 towardLCD display panel 14. Any remaining unsuitably polarized light rays areagain recycled as aforesaid by reflective polarizer 20, diffusing volume18, and rear reflector 16 until the polarization characteristics of therecycled rays matches that of polarizer 20 so that the recycled rays canpass through reflective polarizer 20 toward LCD display panel 14.

Optional angularly selective light transmitter 22, may be formed bycrossing, at 90° to each other, the microreplicated prism structures ontwo parallel sheets of Vikuiti™ Brightness Enhancement Film availablefrom 3M Specialty Film and Media Products Division, St. Paul, Minn.Light transmitter 22 selectively transmits partially collimated lightrays toward LCD display panel 14 in a direction substantially parallelto normal viewing direction 30, while rearwardly reflecting asubstantial portion of any non-emitted light rays back toward reflectivepolarizer 20 and rear reflector 16 so that the non-emitted rays may befurther reflected (“recycled”) for subsequent emission through lighttransmitter 22.

Layered planar light distribution structure 10 is highly efficient inthe sense that it is characterized by low light absorption losses. Ifrear reflector 16, any material within diffusing volume 18, reflectivepolarizer 20, if present, and angularly selective light transmitter 22,if present, are made of materials that do not substantially absorb thelight emitted by LEDs 12, almost all light rays emitted by LEDs 12 intostructure 10 can eventually be emitted through structure 10 toward LCDdisplay panel 14. Unwanted image artifacts are significantly reducedand, in some cases, substantially eliminated, due to the structure'shighly diffuse character.

For clarity and conciseness various elements which can be provided bythose skilled in the art are not described in detail herein. Forexample, a display incorporating an optical structure as describedherein would include suitable driving circuits to cause LEDs 12 or otherlight emitters to emit light. Such circuitry may optionally permit thebrightness of LEDs 12 or other light sources to be individuallymodulated. Any suitable driving circuits may be used including thoseknown to those of skill in the art. Further, a display typically hassuitable driving circuits for operating individual pixels in a displaypanel to modulate light according to image data corresponding to animage to be displayed on the display. Suitable display panel drivingcircuits are also known to those skilled in the field of this invention.Consequently, the driving circuitry for LEDs 12, the driving circuitryfor display panel 14 and other well-understood elements such as powersupplies and the like are not described in detail herein.

Where a component (e.g. a member, part, assembly, sheet, collimator,reflector, etc.) is referred to above, unless otherwise indicated,reference to that component (including a reference to a “means”) shouldbe interpreted as including as equivalents of that component anycomponent which performs the function of the described component (i.e.,that is functionally equivalent), including components which are notstructurally equivalent to the disclosed structure which performs thefunction in the illustrated exemplary embodiments of the invention.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. For example:

-   -   either or both of reflective polarizer 20 and angularly        selective light transmitter 22 may be omitted—satisfactory        results can be obtained by providing only rear reflector 16 and        light diffusing volume 18 in light distribution structure 10. It        is however preferable to include reflective polarizer 20 in        order to increase the light output capability of structure 10,        since unsuitably polarized light rays are otherwise lost. It is        also preferable to include angularly selective light transmitter        22 in order to increase the brightness of light emitted by        structure 10—albeit at the expense of a reduced viewing angle,        since light transmitter 22 partially collimates light which        passes through it. This can be offset by providing an additional        diffuser 32 as shown in FIG. 2 between light transmitter 22 and        LCD display panel 14. Additional diffuser 32 increases the        viewing angle by laterally diffusing light rays which are        narrowly diffused by passage through light transmitter 22.    -   When reflective polarizer 20 and angularly selective light        transmitter 22 are both present they may be arranged in either        order.    -   It is not necessary that rear reflector 16 be perfectly flat.        Rear reflector 16 could be bumpy on a small scale (i.e. have a        surface structure). The faces of diffusing volume 18 should be        generally parallel to one another so that light will be emitted        substantially uniformly from diffusing volume 18.

While a number of example aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truescope.

1. An optical structure for distributing light emitted by light sourcesof a plurality of point light sources, the structure comprising: areflector having an array of apertures for passing light from the pointlight sources into the structure; and a light diffuser for diffusinglight which passes through the apertures, the light diffuseranisotropically scattering light that interacts with the light diffuser.2. An optical structure according to claim 1 comprising an angularlyselective light transmitter for partially collimating, within apreferred angular viewing range, light which passes through the lightdiffuser.
 3. An optical structure according to claim 2 comprising areflective polarizer for polarizing light which passes from the lightdiffuser to the angularly selective light transmitter.
 4. An opticalstructure according to claim 3 wherein the reflective polarizerpolarizes light rays to match the polarization of a display paneloriented parallel to the angularly selective light transmitter.
 5. Anoptical structure according to claim 2 wherein the angularly selectivelight transmitter selectively emits partially collimated light rays in adirection substantially normal to the planar light distributionstructure, and rearwardly reflects a substantial portion of anynon-emitted light rays.
 6. An optical structure according to claim 1wherein each of the apertures is sized, shaped and aligned to match thesize, shape and alignment of a corresponding one of the point lightsources.
 7. An optical structure according to claim 1 wherein thereflector comprises a highly reflective outward surface.
 8. An opticalstructure according to claim 7 wherein the outward surface of thereflector is at least partially specularly reflective.
 9. An opticalstructure according to claim 1 wherein the light diffuser has ananisotropic scattering coefficient such that light rays which aresubstantially parallel to a direction normal to the planar lightdistribution structure are more intensely scattered than light rayswhich are substantially perpendicular to a direction normal to theplanar light distribution structure.
 10. An optical structure accordingto claim 1 comprising a plurality of layers of a thin, weaklylight-scattering sheet material within the diffuser.
 11. A displaycomprising an array of point light sources, a display panel, and anoptical structure according to claim 1 disposed between the point lightsources and the display panel.
 12. A display according to claim 11wherein the display panel comprises an LCD display panel.